As such image forming apparatuses, printers employing inkjet recording systems, plotter, etc., are known. During printing, a conventional inkjet image forming apparatus conveys a sheet a predetermined distance. Then, a print head is moved in a direction perpendicular to a sheet conveying direction (carriage drive), and ink droplets are discharged from the print head to perform image forming. In order to prevent the raising of the sheet and to stabilize printing quality during sheet conveying and during carriage driving, generally, a plurality of suction holes are formed in a platen arranged on a face opposite a print head, and printing is performed while the sheet is drawn down by suction. This sheet conveying and carriage driving is repeated a required number of times, and image forming is completed (see, for example, Japanese Patent Laid-Open Publication No. 2003-326779 (pp. 3 to 6)).
In an image forming apparatus (inkjet recording apparatus) employing the above described inkjet recording system, sheets to be conveyed are limited, and the setting of a suction force relative to the tear-resistance of a sheet or for preventing the raising of the sheet is important. Especially for an image forming apparatus that discharges solvent ink to perform printing, a medium such as vinyl chloride resin film or tarpaulin paper is employed as a sheet. Such media tend to have low tear-resistance and to be easily folded. Further, as a feature of such materials, these media tend to easily stick to paper guides or to platens.
When these media are employed, a sheet that is conveyed is adversely affected by the force of the suction used to prevent raising, and correct conveying of the sheet across the platen is disabled. In portions other than near the conveying rollers, the sheet remains stuck to the platen or the paper guide, so that the sheet is conveyed only near the conveying rollers. As a result, the sheet is raised at the platen and is folded, and the carriage and the sheet contact each other and cause a jam, which may destroy the head.
In addition, in the solvent inkjet recording apparatus, frequently, a sheet heater is provided for the platen or the paper guide and heats the sheet in order to dry the output material after the printing has been completed. In this case, the sheet heated by the heater tends to become softer and to lose tear-resistance, so that sheet conveying is more difficult.
Therefore, one objective of the present invention is to provide an image forming apparatus that enables stable sheet conveying of a medium that has low tear-resistance, tends to stick to something and is difficult to convey.